$59.00
LIBRARY
FOCUS ON:
BOB PEASE
ON ANALOG VOL. I
A compendium of technical articles
from legendary Electronic Design
engineer Bob Pease
Copyright © 2015 by Penton Media,Inc. All rights reserved.
ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
CONTENTS
2|Welcome
3|Remembering Bob Pease
6|What’s All This Designer Stuff, Anyhow?
11 |What’s All This Analog Engineering Stuff, Anyhow?
17 |What’s All This Technical Reading Stuff Anyhow?
25 |What’s All This Transimpedance Amplifier Stuff, Anyhow?
33 |What’s all This Frequency To Voltage Converter Stuff, Anyhow?
38 |What’s All This Capacitor Leakage Stuff, Anyhow?
41 |What’s All This Noise Gain Stuff, Anyhow?
44 |What’s All This Current Limiter Stuff, Anyhow?
49 |What’s All This Output Impedance Stuff, Anyhow? Part 1
52 |What’s All This Output Impedance Stuff, Anyhow? Part 2
55 |More Resources From Electronic Design
electronicdesign.com
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EDITORIAL
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The Electronic D
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EDITORIAL
By Joe Desposito
former Editor-in-Chief | Electronic Design
Originally published June 2011
Bob Pease Remembered For
Pease Porridge And A Whole Lot More
I
t was late on Father’s Day, around
11:55 p.m., when I finally got around
to writing my editorial for this issue,
which was due the next day. I had
some ideas of what I wanted to write
about and was busy gathering information. One piece of information I
needed was in an e-mail I had received
a couple of weeks ago, so I launched
Microsoft Outlook.
Rather than going straight to the e-mail
I needed, I started skimming the e-mails
that had come in over the weekend. The
first one I came across was a notice that
my niece had wished me a happy Father’s Day in a Facebook post. I clicked
on the link, got on Facebook, and sent a
reply. Then I went back to Outlook and
continued skimming the e-mails. One
subject line stopped me cold. It said,
“Bob Pease Killed in Car Crash.” I read
it in stunned disbelief. I looked at the
sender: Paul Rako, the analog editor
from EDN. It must be true, I thought. Horrible news, but true.
In part, Paul said that Bob was killed
when his car left the road as he drove
from Jim Williams’ memorial service yesterday. It’s doubly unfortunate that two
of the greatest analog minds in the business passed in the same week. As it was
earlier on the West Coast, Don Tuite was
online and sending e-mails to our staff.
He sent a link to a San Jose Mercury
News article: “Driver, 70, Dies in Saratoga Crash.”
The short article stated all the facts:
A 70-year-old San Francisco man was
killed after his car hit a tree in Saratoga
on Saturday evening, according to the
California Highway Patrol. The man was
traveling eastbound on Pierce Road at
an unknown speed when he failed to
negotiate a curve to the left at about
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Bob Pease Remembered
5:45 p.m. The driver’s 1969 Volkswagen
Beetle veered to the right off of the roadway and crashed into a large tree on the
right shoulder. The man was not wearing
his seatbelt and it appears he was killed
instantly.
Everyone who reads Pease Porridge
knows that Bob drove a 1969 Beetle.
Bob brought it up many times over the
years and just recently in a popular column about unintended acceleration.
Everyone knew this because Bob was
unrivaled as a columnist in this industry.
Though he was certainly an analog guru
who could write about the nuances of a
very difficult subject area, he also talked
about everyday (and not so everyday)
life situations.
Bob told me many times that his column was about thinking. Whenever he
tackled a topic, he essentially welcomed
readers into a dialogue about how to
properly think about that topic, at least
from Bob’s point of view. But, if you wrote
to him, he would always consider your
point of view as well and tell you what he
thought.
Bob was greatly saddened by the
death of Jim Williams. As you may know,
Jim died recently after a massive stroke.
During the following days, Bob corresponded with me about Jim. In one email, which I think says a lot about the
way Bob thought and lived, he said: “Jim
did write more huge SYMPHONIES of
big Apps. Systems. I wrote more small
ones. We always had very similar ideas
on helping Users with Analog problems:
We never turned down a request for
Analog help. We agreed on that.” Then,
Bob said: “I am very SCRUPULOUS
about taking my 5 or 7.5 mg of Coumadin every day. I don’t know if Jim was on
Coumadin. Coumadin = Warfarin = Rat
Poison, good for preventing Strokes.”
Bob also asked me to reprint part of
a column on doctoring that he had written years ago about how to tell if someone was having a stroke. This particular
column, “What’s All This Floobydust,
Anyhow? (Part 14),” contains a section
called DOCTORING STUFF, PART 4C—
STROKE DIAGNOSIS.
In Bob’s grief about Jim, his first
thought was to let the readers of this
magazine know how to tell if someone is
having a stroke. He starts off the section
of this column by writing: “Many people
know that in case of a heart attack or
stroke, it is very important to get the victim to medical care very quickly, within
much less than an hour. But what do we
know about diagnosing such an unhappy person?” And he goes on from there
to impart his knowledge on this topic
and hopefully help someone save a life
someday.
Unfortunately, we now have to say
goodbye to Bob and all the wisdom he
so generously shared over so many
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Bob Pease Remembered
years writing for Electronic Design. He
was a tremendous talent and we will
miss him greatly. You can find his latest
column in our July 7 issue.
You won’t be surprised to learn that he
had the drafts for number of future columns in the works in addition to his popular “Bob’s Mailbox” collection of correspondence. We will work with his family
on bringing those columns to you in the
future. We think he would have wanted
you to read them. And finally, wherever
he may be right now, I’m sure he’s thinking about writing, “What’s All This Car
Crash Stuff, Anyhow?” n
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?
What’s All This
Designer Stuff,
Anyhow
Originally published January 1995
A
little while ago, I ran across an article
in Trains magazine (Oct. 1994, pp. 4649) about “Martin Blomberg, designer
extraordinaire.” What did Mr. Blomberg
design—a snazzy engine paint job?
A new Diesel engine? Nope. He
designed the “Blomberg truck.” It’s an improved
frame that goes under each end of an engine—a
wheel assembly with two axles, four wheels, and
two big 300-kW electric motors. The truck was
designed by Mr. Blomberg for the Electro-Motive
Corp., a subsidiary of General Motors Corp.,
in 1937. This was first used on a Demonstrator
freight locomotive, which logged 83,000 miles on
20 railroads in 1939-1940, and was acclaimed
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What's All This Designer Stuff, Anyhow?
as a great success wherever it went.
Blomberg’s truck is still in use today on
most of the GMC freight locomotives.
I think that’s pretty good for a piece of
1939 engineering.
Why was the Blomberg truck considered such a good design? Well, previously, diesel passenger engines ran on
flexible six-wheel trucks with two traction
motors. The middle axel carried weight,
but was not available to be powered.
The early Diesel-electric engines had
rigid four-wheel trucks and were suitable
for yard switchers, not for long-distance,
high-speed freight hauling. They had as
good stability and ride comfort as steam
freight engines, but not much better. In
the late 1930s, Diesel engineers had
to plan new freight engines that could
move freight at fairly high speeds—but
put out more power and more tractive
force than any passenger locomotive.
In other words, they had to provide the
advantages of all the old, slow diesel
switcher engines—and the advantages
of the sleek fast passenger engines—yet
put out a lot more power.
Mr. Blomberg’s new truck design did
that. It put all the weight on the driving
wheels without any unpowered axle.
He provided a firm but flexible suspension, putting all the weight evenly on all
four wheels. According to the article,
“He designed the truck with a minimum
of assistance, for there were no others
at EMC acquainted with truck design.
Strain gauges were crude, and finite element analysis was decades away. The
designer himself had to rely on fairly
simple calculations and a strong sense
of mechanical aptitude, with perhaps a
little luck added.”
So, what kind of man was Martin
Blomberg? “He could be very courteous, but his standard answer, if anyone
approached his ‘territorial waters’ with a
suggestion was, ‘Ve do it my vay.’” Does
that sound like anybody you know?
Recently, I was trying to write down
the Job Description for a Product Engineer. Just for the heck of it, I also wrote
down, for comparison, a definition of a
Design Engineer. There’s a lot of similarity, a lot of overlap, except for one major
difference: A good Design Engineer not
only has to put together a lot of circuit
functions, using known designs, but he
also must know when existing designs
aren’t good enough, and when and how
to make new circuits.
What is a “Designer”? Is he/she a person who designs circuits? Wears flamboyant clothing and plans the décor for
a house or an office—or a locomotive?
Designs the transmission or the grille
for a new car? Well, yes, a designer can
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What's All This Designer Stuff, Anyhow?
be any or all of these things. But after
nobody learns a lot of circuits—or a lot
you learn how to analyze things and
of anything in detail. In the old days, a
prove the feasibility of a design, it’s also person had to LEARN a lot of circuits to
of great value to be able to invent new
get his amateur radio license. But these
circuits—new designs. To do that, you
days, learning about and understanding
have to be familiar with lots of old dea number of radio circuits—receivers
signs. You have to know what each old and transmitters—is no longer required.
design did well, and what
When I began to get init did badly. Basically, you
Guys like Bob Widlar, terested in circuits around
just have to KNOW a lot of
my senior year, when I got
Bob Dobkin, and many out of the Physics course
old designs.
When I started designothers knew how to and into EE, I got an aping discrete-component
petite for learning a large
innovate.
I
can’t
say
I’m
op amps in 1061, I studied
number of circuits. I really
every circuit I could lay my
in the top echelon of got engrossed in this, as
hands on—voltage regulaif it were a hobby. Only if
innovators,
but
I
know
tors, vacuum-tube circuits,
you’re REALLY interested
transistorized circuits. I
how to get a job done. in a field, like the most enused good old, proven cirthusiastic hobbyist, will you
cuits when I could and invented new
learn all the history of design, so you
ideas when the old ideas weren’t good can tell when you have to break new
enough. As I said in my first column
ground and invent new circuits. Most
four whole years ago, “computers may students never get that interested—they
be able to help you optimize a given
have to take a smattering of course on
design, but it is not necessarily helpmany different subjects to be able to
ful when the old design is not good
graduate. That’s not the kind of intensity
enough.” Guys like Bob Widlar, Bob
you need to be a good designer.
Dobkin, and many others knew how to
In addition, when you’re innovating,
innovate. I can’t say I’m in the top eche- you must be good at circuit analysis
lon of innovators, but I know how to get so you can see problems, drawbacks,
a job done.
and limitations of new circuits. You must
Note, you can learn a number of
be good at this, by intuition or by quick
things in school, in college, but almost
analysis, so that you don’t have to drag
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Designer Stuff, Anyhow?
every new circuit through a long, slow
Spice analysis. Besides, as I’ve mentioned many times, Spice might tell you
a circuit would not work even when it really does. You can’t run every potentially
good idea through Spice.
Recently, I was trying to design a
low-power R-S flip-flop in a bipolar circuit. I invented about four designs.
Each one looked pretty good until I really did a one-minute pencil-and-paper
analysis. Blah. They would not work.
I finally asked a buddy for advice. He
told me about a circuit he had used.
We couldn’t use that one due to power-supply limitations. But, I modified
that circuit to go with some low-voltage
compactors I already had and everything clicked. I haven’t even bothered to
breadboard it, nor to Spice it, because
I can see how it has to work, perfectly,
using just a little pencil and paper.
Now, I try to avoid saying good or bad
things about a competitor, so I won’t.
But I will mention something about a
new design made by a kid engineer
at National about 20 years ago. Once
upon a time, we had an LM109, designed by Bob Widlar, and it was good
for about 1.5 A at 5 V. We also had an
LM140, designed by George Cleveland. It had a much smaller die size and
would do at 1.5 A at room temperature,
but only 1 A at all temperatures.
It was designed to be competitive
commercially, especially when competing with Fairchild’s UA7805. Unlike the
LM109, the LM140 came in several different voltages—5, 6, 8, 9, 10, 12, 15,
18, and 24 V. We knew that, obviously,
customers needed various different output voltages.
About 1974, Bob Dobkin asked Brent
Welling, the Manager of Marketing for
Linear, “What if you could have an adjustable voltage regulator that you could
adjust to any voltage from 1.2 V to 40
V?” Brent asked, “Will it cost more to
manufacture than an LM140?” Answer,
yes. So Brent said, “Well, then there
will not be any possibility of significant
sales.”
I’m not exactly sure how we did it, but
in those days at NSC, we didn’t have
“teams.” We didn’t exactly have consensus. We didn’t always have harmony
and sweetness and light. But we had
ways of getting parts out. I wonder if we
wouldn’t be better off if we could reconstruct that..
Anyhow, Bob Dobkin convinced us to
get the LM117 into production, and it
was a big winner. Of course, the LM117
has never outsold that LM140 in number
of chips sold, because the LM140 was
quite adequate for simple requirements.
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What's All This Designer Stuff, Anyhow?
But the LM117 made a good profit because it made lots of customers happy
in critical applications.
Why? Well, it had a BIGGER power
transistor with more ballasting. So, you
could get more power out of it, at higher
voltages, without blowing it up. Some of
NSC’s competitors tried LM117’s good
control circuit. But they used a smaller power transistor so they could get a
cheaper, smaller die. As a result, their
“117s” blew up with minimum abuse. So
the LM117 sold well, because it could
really do things that competitors could
not do.
Here’s another quote from the Trains:
“Blomberg’s truck design has been able
to withstand the far greater demands
imposed by today’s locomotives with
their higher horsepower and increased
tractive effort. It has been said that he
was not as cost-conscious as he should
have been, that his designs were heavier than necessary, and that he would
not listen to others. Many people would
gladly plead guilty to such criticisms if
their designs could be as successful as
Blomberg’s.” Ain’t that what Dobkin did
with his LM117?
What else should a designer do?
He must not just think of how to meet
specs—although that’s important too.
He/she must think like a customer (a
FOCUS ON: BOB PEASE ON ANALOG
user) and see what will really make them
happy—and avoid things that would
make a customer unhappy. You have to
put on a marketing hat to see what features will appeal to the customer. You
have to close your eyes and think—“how
can I write a data sheet that will have sex
appeal to engineers?” And if you can
think of an advertisement, so much the
better!
You have to be careful not to build in
features that are excessively hard or expensive to make in production. You have
to plan that any necessary test can actually be done without a lot of wasted time
and expense.
Obviously, in every field—in styling a
car, in decorating a room or a building,
in designing a nine-ton two-axle truck to
carry freight engines—you have to think
about all of these facets. A good designer neglects none of these. Yeah, a truism. But it is true.
Recently, some outstanding engineers
were described by the publication American Heritage of the Invention & Technology as “bold, self-reliant, independent,
secure, powerful, daring, resolute, and
sometimes, arrogant and overbearing.”
So, what’s new? n
RAP/Robert A. Pease/Engineer
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?
What’s All
This Analog
Engineering
Stuff,Anyhow
Originally published October 2008
Don’t get dismayed by the rise of digital
technologies. Analog engineers are still critical
members of any design team.
F
or many years, aficionados of digital
circuits and computers have bragged that
their rapid advances will leave all analog
circuits lying in the dust. The analog
business is shrinking, at least compared to
the success of digital computers. Moore’s
law has made sure of that for many years. The
tiny transistors are smaller and faster than ever,
even if they can’t stand off 5 V (Fig. 1).
The efforts of a team of digital men who engineered and fit together a few hundred transistors
has given way to automated schemes to assemble
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What's All Analog Engineering Stuff, Anyhow?
many thousands, many
Some sensors put out a
millions, and now billions
signal that can be acquired
of transistors. Boy, every
directly by a fairly simple anacircuit must cost millions of
log-to-digital converter (ADC)
bucks! That means every
that interfaces to the sensor.
microprocessor or systeBut, typically, a high-permon- a-chip (SoC) must
formance ADC needs an
be very profitable! Right?
anti-aliasing filter to prevent
Wrong. It ignores the
the sampling from turning
fact that several thou1. The electronics industry has high-frequency noises and
sand digital transistors
evolved as packages have
spikes into low-frequency
may now sell for less
gotten smaller and smaller. For “artifacts.”
than a penny. Digital IC
example, the 14-pin dual-inline
What kind of filter is needmakers must give away package (DIP) is now today’s
ed, and how many dB of
a million transistors to
micro-SMD.
attenuation are needed at
make a few bucks. This
the sampling frequency?
isn’t necessarily true for analog circuits. A The person who designs that filter has to
couple dozen years ago, we handed out complete the filter engineering in the ansome license plate frames in Silicon Val- alog domain. You can’t do it with digital
ley that said “One good op amp is worth signal processing (DSP). You must have
1000 microprocessors.” We still believe
a good analog filter before you get the inthat!
formation converted into signals that can
A digital computer can do some things be processed by DSP.
by computing the facts it is told. But to
Now, in theory, it sounds like an ADC or
perform a useful function, it often needs
digitalto- analog converter (DAC) will be
a good bit of analog information. It needs designed by one analog group and one
to get information from the world or from digital group that sit down, shake hands
its user. It also needs to get this data from across a table, and figure out how to
sensors, where the information is changet their circuits to do a handshake, too
neled through analog preamps and/or
(Fig. 2). But in practice, most high-perfilters. Usually, analog engineers have to formance ADCs are designed by analog
engineer these channels. A brute-force
engineers. They have to figure out how
approach generally doesn’t work.
all of the signals and waveforms will get
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What's All Analog Engineering Stuff, Anyhow?
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FOCUS ON: BOB PEASE ON ANALOG
along without causing
on such a low-voltage
trouble.
chip.
Yes, they do have
So, these functions
to handle some digital
are often being added
signals, but that’s not
as external chips. They
too hard. We analog
don’t hurt the yield, as
engineers know how
they might if they’re
to handle “digital” sigintegrated on the main
nals without excessive
chip. They don’t delay
bounce or overshoot!
release of a system
We know how to de2. It may look like a rat’s nest, but this
that is nearly finished.
sign and lay out trans- voltage-to-frequency converter was
Mindless attempts
mission lines, terminat- built over a ground plane using an
at further integration
ed as needed.
LM331 and a couple of optional sizes
have, in many cases,
Sure, some ADCs
of filter capacitor. That’s how I usually
been replaced with disare integrated onto the wire up a breadboard. I never use a
integration.
main SoC, but these
solderless breadboard. I built it to
There are still appliare mostly the low-per- confirm that the LM331 still works just cations for digital comformance (slow or
as well as it did when I first invented it
puters, where most of
low-resolution) ones.
31 years ago.
the work is just compuHigh-performance
tation. When the comones are usually done off-chip. They are puting is all done, after a few hours, the
often more costeffective or time-effective. computer spits out the answer: “42.” But
Thus, analog circuits are also needed
these days, there’s often a lot of interacalongside modern microprocessors. En- tion between the user and the computer.
gineers used to try to add a lot of analog
Sensors are needed. And, the sensor
functions into the processor. But smaller needs an ADC to convert the variable
feature sizes, faster logic, and low oper- (force, position, pressure, temperature)
ating voltages have forced DIS-integrainto a digital format so the processor
tion because a decent audio amplifier,
can figure out what to do with that inforlow-noise preamp, bandgap reference,
mation.
high-resolution ADC, or anti-aliassing
Temperature is one of the parameters
filter can’t be made (profitably or at all)
that SoCs often try to sense. Sometimes
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What's All Analog Engineering Stuff, Anyhow?
the system wants to know the ambient
temperature. Sometimes it also wants
to know about the processor’s temperature to help prevent overheating. It isn’t
impossible to do this with the temperature computation done on the main chip.
However, it’s usually better, cheaper, and
easier to do it accurately with an external
(disintegrated) temperature-measuring
circuit off the main SoC.
This is often done with a remote diode
temperature sensor (RDTS), which can
sense temperature using almost any kind
of stable diode or transistor. It can even
sense the temperature of one transistor
built into the middle of the main processor and, thus, protect it. Of course,
detecting the ambient temp requires an
off-chip sensor. Why not put the temp
measuring function off-chip?
modern CMOS circuits need to run with
a regulated supply.
Many low-dropout (LDO) regulators
can run accurately on low voltages and
regulate a battery source down to a lower voltage, but they generally aren’t very
efficient. LDOs often waste as much
power as they put out. So while they are
quite useful in some cases, they won’t
let you run your cellular phone for a long
time. I mean, which would you buy—a
cell phone or computer that runs for
three hours or six hours?
That’s why we need switch-mode regulators to get good efficiency. Aha! A
switcher has drivers and power MOSFETs that turn on and off—a bang-bang
controller. That must be a good digital
application!
But not at all. While the nominal voltage
levels seem to be ones and zeros, the
Voltage Regulator Stuff
actual output voltage depends precisely
on the time ratio or duty cycle of those
It’s quite true that many CMOS ICs can apparently “bang-bang” signals. The
run on a wide range of power-supply
duty cycle of these signals is an analog
voltages. So, you can get some things
function and is controlled by an analog
done by running them on a small battery. controller. So all computers these days
But modern high-performance CMOS
run on regulated power, regulated by
circuits run fast on a low-voltage supply. analog switch-mode controllers—controlIf the battery voltage runs low, the CMOS lers designed by analog designers.
runs slow, and the timing may suffer.
The only digital things in those conIf the battery gets too high, the CMOS
trollers are the techniques by which the
starts to break down and overheat. So,
fast duty-cycle signals are driven. And
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FOCUS ON: BOB PEASE ON ANALOG
Note that these are analog techniques.
I used to think that LVDS was the
dumbest idea in the world. Why would
anybody buy that? Then, after a long
time, I began to realize that if LVDS were
so stupid, nobody would buy it. Yet these
LVDS drivers, multiplexers, and receivers
were selling well, and the business was
3. Op amps give designers lots of flexibility
even expanding. It was I who was kind of
and can even lead to applications that the
dumb.
op-amp manufacturer never intended.
People wouldn’t buy them if they
weren’t useful, cost-effective, and valueven these need analog techniques to
able, even if I was too stupid and slow
help them behave and save power. On
to see the value. After all, it was my old
a good day, analog engineers are wild
colleague Jay Last, one of the original
men about saving power. Sometimes we Fairchild Eight, who observed, “The only
are very good at it.
valid market survey is a signed purchase
Even a digital computer may need an- order.” A lot of analog circuits are sold
alog circuits, such as low-voltage differthat way. The customer knows what the
ential signaling (LVDS), to transfer a lot of ICs are useful for, even if the maker does
data from place to place. All you digital
not.
engineers now know that you can’t just
An operational amplifier is called that
feed some full-size digital signals at high because it can perform just about any
speed to a different location, like on a
operation according to what feedback
backplane, or on flex cabling, at flat-out
components you put around it. The beauspeed.
ty of the op amp is exactly what things
You have to engineer (using refined
the customer can (and does) think to do
techniques) the signals into small, balwith an amplifier, or a regulator, that we
anced (push-pull) differential signals.
never thought of or told him how to do
Then, you have to put the signals on
(Fig. 3).
carefully laid-out transmission lines. And,
New applications are invented every
you have to use nice little preamps to
day! Often, the customer does that inrecover the signals at the required place. venting. Sometimes he tells us what he’s
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All Analog Engineering Stuff, Anyhow?
doing or asks if it’s okay. But usually he
is too busy to tell us, and sometimes he
really doesn’t want to tell anybody.
Analog drivers are also needed for
backplane drivers and display drivers in
computer displays. Getting a lot of info
up there needs careful analog planning,
not just a lot of wires. There are still a lot
of applications that depend on layout
to get a circuit to work well. And, many
parts of a good layout depend on analog
engineering.
Some wireless techniques rely on a lot
of digital codes. That’s very true. But to
pick these codes out of the air, the preamps need analog techniques, with good
RF preamps. They need mixers and AVC
circuits to avoid overloading. I’m not a
very good RF engineer, but I know that
the art of RF engineering isn’t simple. If a
digital engineer can do it, that is all very
good, but then we should properly call
him an RF engineer.
do something else.
I’ve never been tempted to do that. The
analog business is almost always challenging. Our work often involves challenges where computers or simulation
cannot help us. But experimenting often
can. And thinking often can.
In the last year, some of my buddies
showed me some excellent audio amplifiers with nonlinearity down near or below
1 ppm. I studied around and measured,
and I found these amplifiers were actually 10 times better than the engineers
thought they were, using advanced analog measurement and analysis techniques. I also cheated by bringing in
various resistor and series R-C networks.
There are still things you can do with Rs
and Cs that are fun ways to solve problems, and they are not obvious!
If you’re an analog engineer, don’t
jump off any bridges because somebody
tells you that Moore’s law will put you out
of business. (In actuality, I have indicaWhat’s Next?
tions and proofs showing that Moore’s
law is the one in trouble.) Stick together
Are we analog engineers worried about with your analog buddies and keep solvthe analog business dying out? I don’t
ing tough problems. I think you will be
think so. Every year, we are confronted
rewarded. We’re going to keep on havwith more work and problems than we
ing a lot of fun— analog circuit fun. n
can do in a year and a half or two years.
RAP/Robert A. Pease/Engineer
Most of it is profitable. A lot of it is fun! If I
weren’t having fun, I would go away and
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ELECTRONIC DESIGN LIBRARY
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?
What’s All This
Technical
Reading Stuff,
Anyhow
Originally published March 1994
R
ecently a young engineer, Barrett L. of
Poughkeepsie N.Y., wrote me a letter. He
not only ordered one of my books, but
he asked my advice and opinion: “I’m a
recent E.E. graduate, and if you could
please supply me with a list of suggested
reading (i.e. books, periodicals, columns) it would
be greatly appreciated.” Well, I had to reply:
“Dear Mr. L.--Darn it, you are the second person this week to ask me this question, and while
I was stupid enough not to get the message the
first time, I won’t miss the message twice. Fortunately, there are several good books I can recommend.
You have not gone wrong in ordering my book
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Technical Reading Stuff, Anyhow?
on Troubleshooting Analog Circuits1. I
try to keep things light and breezy, with
REAL examples. And I also try to minimize the platitudes, as you have noted
that nothing gets me TICKED OFF faster
than meaningless platitudes. I think real
engineers get their education from EXAMPLES, and I try to write accordingly.
A second related book, from the same
publishers, is Analog Circuit Design2,
edited by Jim Williams. It consists of
about 33 chapters from 23 authors. It,
too, is educational in terms of EXAMPLES. Some of the chapters are so-so
(in my opinion) and then they get better
and better, up to great. The chapters
by John Addis and Derek Bowers are
great. I wrote two good chapters.
NOW, if you laid hands on this book
you might decide that you wanted to
buy it, OR you might read a few chapters and then put it back on the shelf.
So get your librarian--your town librarian or your company librarian--to order
it. If you ain’t on good terms with your
librarian, you should be. After the librarian buys it, it can be shared by a good
number of people.
Nextly, there’s Horowitz and Hill’s The
Art of Electronics3. Any bookstore will
order it for you. It’s in its second edition----very popular, fun to read, good
insights. Your company librarian would
be wise to buy this one if there are more
than six engineers in your company.
Maybe you have read this already. It’s a
good reference book. If there are older
engineers who went through school before this book came out, or technicians,
they may find this of high value.
At this point, I gotta ask you, Barrett,
where are you coming from? What good
electronics books have you read? Did
you take a lot of electronics courses?
How many analog, how many digital,
how many software? I can assure you, I
can give you ZERO advice on software.
I have written a couple of successful
chunks of BASIC, but my opinions are
worthless. I can’t give you much advice
on digital circuits, and books thereon,
but I’ll ask my friends.
Next question for you--in which direction do you think you are going?? Analog, ADCs, or systems? Low power?
High speed? Lotsa processors and a
little analog? Just trying to get a broader
education? That will make some difference. Or, if you’re not sure where you’re
headed, well, the broad perception from
reading a lot will be good for you.
OF COURSE you gotta keep reading Electronic Design.4 It gives you a
good clear presentation of some of the
newer ideas, circuits, concepts, and
trends, with intelligent guys from the
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Technical Reading Stuff, Anyhow?
industry trying to make good explanaAmplifiers (second edition)7 . I’m selltions----trying to play teacher. One item ing these for $53. This used to sell for
of advice: Read and mark the hell out of $113 from Elsevier Scientific, and was
any stories that are interesting. Xerox or worth it. Now at $53, it’s a bargain. Secross-index the stories that are of good rious, thoughtful. NOT your FIRST priminterest, but don’t throw the magazines er on op amps, but any serious user of
away for at least five years. You can go op amps should read this about once
back in several years and see what’s in- a year. You’ll learn something new evteresting, what’s trivial, and
ery time. However, that
In
which
direction
what’s pass‚. Note, a fivestatement is true for almost
year stack of ED just fills
do you think you are every one of these books:
two “Xerox-paper boxes,”
All of these books are well
going??
Analog,
ADCs,
about 2.5 cubic feet....
worth rereading every year
or systems? Low
Read hobby magazines
or two.
5
----Popular Electronics or
Now, darn near your first
power? High speed?
similar publications. Some
primer on op amps is Tom
Lotsa processors and Frederiksen’s Intuitive IC
of the stuff is trivial, but
8
that’s OK----sometimes it’s
a little analog? Just Op Amps. Heck of a fine
good to read stuff where
book. Where is it sold? Tom
trying
to
get
a
broader
you’re smarter than the
still has some to sell. If you
authors, and you can see
want to use op amps, you
education?
if they’re doing something
want to do it intuitively. No
stupid.
fancy formulae, no matrices, no TaguJames Roberge of MIT wrote an exchi optimization. Just common-sense
cellent book about op amps in general, engineering with good intuitive insights.
and about the LM301A in particular.6
Tom’s an excellent teacher for that.
The first half explains how the ‘301 chip
The NEXT primer on op amps is
was engineered; the second half goes
NSC’s linear databook on op amps.9
into how you can apply a ‘301. I asked
The history of op amps was written
James and he said it’s still in print. It’s a there.
very good read.
NOW, Analog Devices’ op-amp dataThe advanced, expert book on op
book might be almost as good as NSC’s,
amps is by Jiri Dostal----Operational
but they usually don’t include device
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Technical Reading Stuff, Anyhow?
schematics. If you want an AD databook, call them up.10 You can request
and study databooks from anybody in
the whole electronics industry, but I find
the ones that do include schematic diagrams (of the devices themselves) are
the most useful and educational.
NEXT, NSC’s Linear Applications
Handbook.11 Some of the better early
linear IC applications breakthroughs
have gone into there. It’s indexed pretty
well, so you can find what you want.
NSC’s databook on regulators and
power ICs12 is quite good----a useful
basic reference book. Also falling into
that category is the Data Acquisition
Databook13, which covers DACs, ADCs,
voltage references, and temperature
sensors. Another is the Linear Applications-Specific ICs Databook,14 which
includes several strange but useful ICs.
The list price for these NSC handbooks is usually about $10 or $15 each.
But our marketing guys agreed that we
ought to make a special offer to readers
of Electronic Design----serious engineers. If you want any or all five of those
linear databooks, just call and ask for
the set.15 Not a bad deal.
If you ever have to do “optimization” or quality problem-solving, buy
the books that cite “Taguchi” in them
the fewest number of times. I just got
around to reading Keki Bhote’s book16,
full of excellent common sense. I started
out skeptical, but eventually decided I
really liked his style and common-sense
approaches. Hans Bajaria has a good
book.17 So does Forrest Breyfogle.18
And Diamond’s book19 is good, too.
Just keep away from books by Genichi Taguchi, and all his friends who tell
you how great it is to use orthogonal
matrices to solve any problem and you
don’t have to think... (if you know what I
mean, and I think you do).
Recently, I was introduced to a new
book by Dennis Feucht, Handbook of
Analog Circuit Design.20 It really covers
lots of the things a designer needs to
know about analog circuits. I’m favorably impressed, and I recommend it. It
has good chapters on wideband amplification, precision amplification, feedback circuits, frequency compensation,
signal-processing circuits, etc. There’s
some overlap with Hill and Horowitz, but
that’s good, not bad.
Another good new book is Highspeed Digital Design, by Howard Johnson, subtitled A Handbook of Black
Magic.21 I thought it was pretty good,
so I loaned this to one of our best digital/mixed-signal designers. I was a little surprised when he returned it right
away. He said that was because he had
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Technical Reading Stuff, Anyhow?
gone out and bought several copies
for the guys in his group. It treats the
gray area between signals that are digital, and the analog aspects that are so
important when you want your digital
buses to behave at higher and higher
speeds----not a trivial task. This book is
there to help, with serious advice and
good philosophy. You ain’t gonna get
much of that anywhere else these days.
One of my old fans, Reg Neale, recommended a book on ESD, ESD from A
to Z, by John Kolyer.22 I discovered the
book in our library, read through it, and
found a number of thoughtful observations. Just as my book is the “best” book
on Troubleshooting, so this book may
well be the “best” on ESD, and partly for
the same reason----it’s the~ only one.
When I wrote about Ground Noise
in June, I recommended the excellent book by John Barnes, Electronic
System Design,23 and I still do. Also,
a friend recommended Noise Reduction Techniques in Electronic Systems,
by Henry Ott.24. Its second edition just
came out, and it, too, covers many topics that aren’t taught in schools. If you
work with RFI and EMI in real systems,
you’ll probably profit by buying and
reading both of these books.
Recently, Barrett, a reader asked me
what book can I recommend to teach
about designing printed-circuit boards?
I asked several friends----no ideas, no
recommendation. Maybe some of our
readers can recommend a book? There
ought to be something beyond the literature of the pc-board-design software
people...
Also, you and other readers who recall the question and debate of the value of a Bachelor’s Degree versus an
Associate’s Degree in Electronics Technology may be interested in a new book
by Joel Butler, High-Technology Degree
Alternatives.25 He observes that you
don’t have to go to school at night for a
dozen years, nor drop out of work and
pay tuition for four years. He has several
suggestions on how you can get credit
for school courses and work you have
already accomplished. He recommends
a list of a few dozen schools where you
can apply by mail. This sounds kind of
unlikely, except Mr. Butler points out that
these schools actually are ACCREDITED, which isn’t a trivial statement.
One last “book,” actually some stories
on floppies, was sent to me by Geoff
Harries, a reader located in Munich,
Germany. They are Science Fiction, sort
of high-tech and time-travel, and good
historical stuff, too. He’s still trying to get
a publisher. Meanwhile he gave me permission to sell you his book, ChronDisp
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Technical Reading Stuff, Anyhow?
I (about 700 kbytes) on a high-densidoes or makes, but let’s assume there
ty IBM-type floppy for about the same
are PRODUCTS somewhere. When
price as a paperback book. I will repa- I joined Philbrick in ‘61, I studied the
triate all proceeds to Geoff. I enjoyed
heck out of all its schematics and data
the book, and I recommend it to you,
sheets, op amps, and analog computtoo. It’s kind of fun to sit there in the eve- ing products. Then I would say to myning, hitting “Page Down,” again and
self, “Why did they do that?” I surely
26
again, reading Geoff’s stories.
couldn’t understand everything, but by
Well, there is my list. This
asking all questions that
may not be a DEFINITIVE
Then I would say to came to mind, I got a heck
list, but it’s a good start.
of an education.
myself,
“Why
did
they
Ask your buddies. Borrow
Similarly, when I came
do that?” I surely
some of their hobby magato NSC in 1976, I studied
zines. Also, you will probacouldn’t understand all of the op amps, data
bly want to read one or two
sheets, schematics, AND
everything,
but
by
general-purpose science
layouts. I’m sure you will
magazines. I used to read
asking all questions agree that in an IC, the lay27
Scientific American, but
out can be even more imthat
came
to
mind,
I
got
a year ago I gave up on
portant than the schematic.
them and changed over to a heck of an education. In my first year at NSC, I
Discover.28 Maybe Sci Am
spotted a philosophical
is coming back, but I still
error in the layout of a popthink Discover is excellent. Try Machine ular amplifier. The changes I suggested
Design.29 You can spend a lot of time
caused the yield to go up on the LF356,
reading this, but it’s fun and education- the LF256, AND the LF156, by a facal. You’ll see several kinds of good engi- tor of 2, EACH. That yield improvement
neering.
paid for my first couple years’ salary,
Ah----yes, let’s not overlook the obeven if I had done nothing else.... So,
vious. You should also read your own
read your own company’s literature,
company’s catalog or data sheets. Look and the serious art (schematics, layout,
at what it says to your customers, and
software, or whatever) that’s behind the
look at the circuits “behind the front
scenes.
panel.” I don’t know what your company
NOW you can plainly see that I have
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ELECTRONIC DESIGN LIBRARY
What's All This Technical Reading Stuff, Anyhow?
FOCUS ON: BOB PEASE ON ANALOG
written a whole column around your
request, for pity’s sake! I hope I have
answered the question for you and for
many other guys who are just out of
school and trying to get going----up the
LEARNING CURVE. So, read books and
think of good questions. When you have
answered as many as you can, and you
have asked your colleagues, and there
are still some you cannot answer, write
down some notes and ask one of your
Senior Engineers. He’ll probably be flattered to get thoughtful questions from a
serious student. If you ask reasonably,
he/she may provide some (priceless)
mentoring. Have fun, Barrett!!”n
3. The Art of Electronics (2nd Edition), Paul
Horowitz and Winfield Hill, 1989, 800 pages.
About $60. Cambridge University Press, NY;
(914) 937-9600 or (800) 872-7423. In NY,
(800) 227-0247.
4. Electronic Design, Penton Publishing. To
apply for a free subscription, write to Electronic Design, Reader Service Dept., 1100
Superior Ave., Cleveland, Ohio 44197-8132.
5. Popular Electronics, about $19 per year.
Call (800) 827-0383 or (516) 293-3000.
6. Operational Amplifiers: Theory and Practice, James Roberge, 1975, 678 pages.
About $71. Wiley/Krieger. Call (407) 7249542.
7. Operational Amplifiers (Second Edition), Jiri Dostal, 1993, 555 pages. ButterRAP/Robert A. Pease/Engineer worth-Heinemann, Stoneham, MA. Available
from Robert Pease for $53, including tax &
REFERENCES
shipping. (see address in reference 1)
1. Troubleshooting Analog Circuits, 3rd Print- 8. Intuitive IC Op Amps, Thomas M. Frederiking, Robert A. Pease, 1992, 208 pages.
sen, McGraw-Hill, 1988, 352 pages. Order
Butterworth-Heinemann. Order from Robert
from Thomas Frederiksen, 24705 Spanish
Pease, 682 Miramar Ave., San Francisco, CA Oaks, Los Gatos, CA 95030; $25.00 includ94112. NOW in PAPERBACK----NEW LOW
ing tax & shipping.
PRICE----$25.95 including tax & shipping.
9. Operational Amplifiers Databook, National
2. Analog Circuit Design: Art, Science, and
Semiconductor Corp., 1993. Publication No.
Personalities, edited by Jim Williams, 1992,
400061.
222 pages. Order from Robert Pease, (see
10. Analog Devices Inc., Literature Dept.
address in reference 1) $47.95 includes tax & (617) 461-3392.
shipping; or order it from the publishers, But- 11. Linear Applications Handbook, National
terworth-Heinemann, Stoneham, MA; (800)
Semiconductor Corp., 1994. Publication No.
366-2665 or (617) 438-8464.
400043.
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Technical Reading Stuff, Anyhow?
12. Power ICs Databook, NSC 1993. Publication No. 400062.
13. Data Acquisition Databook, NSC, 1993.
Publication No. 400033.
14. Linear Application-Specific ICs Databook,
NSC 1993. Publication No. 400060.
15. Set of 5 NSC linear databooks----items 9,
11, 12, 13, and 14 above----call the NSC Customer Response Center, (800) 272-9959 or
(817) 468-6811.
16. World Class Quality: Using Design of Experiments to Make It Happen, Keki R. Bhote,
AMACOM, 135 W. 50th St. NY, NY 10020;
(518) 891-5510 About $25.
17. Statistical Problem Solving----A Team Process for Identifying and Resolving Problems,
Hans Bajaria & Richard Copp, 1991, 300 pages. About $45. Multiface Publishing Co., Garden City, MI; (313) 421-6330.
18. Statistical Methods for Testing, Development, and Manufacturing, Forrest Breyfogle,
John Wiley & Sons, 1992; (908) 469-4400.
About $65.
19. Practical Experiment Designs for Engineers and Scientists, William Diamond, 1981,
347 pages. Van Nostrand Reinhold, NY. About
$55. Call (800) 842-3636 or (606) 525-6600.
20. Handbook of Analog Circuit Design, Dennis L. Feucht, 1990, 685 pages. About $65.
Academic Press/Harcourt Brace Jovanovich,
San Diego, CA; (800) 782-4479 or (407) 3452500.
21. High-speed Digital Design (A Handbook
of Black Magic), Howard Johnson and Martin
Graham, 1993. About $47. PTR Prentice-Hall,
Englewood Cliffs, NJ; (206) 556-0800.
22. ESD from A to Z, John Kolyer and Donald
E. Watson, 1990, Van Nostrand Reinhold, NY;
(800) 842-3636 or (606) 525-6600. About $47.
23. Electronic System Design: Interference
and Noise-Control Techniques, John R.
Barnes, 1987, 144 pages. About $48. Prentice-Hall, NY; (800) 947-7700 (out of print).
24. Noise Reduction Techniques in Electronic
Systems, (2nd edition), Henry Ott, 1988, 426
pages. About $65. John Wiley & Sons, NY;
(908) 469-4400.
25. High-Technology Degree Alternatives,
Joel Butler, 1994, Professional Publications,
1250 Fifth Ave., Belmont, CA 94002; (415)
593-9119. About $22.
26. ChronDisp I: The Gun From The Past,
Geoff Harries, 1993, Munich, Germany. Can
be ordered on a high-density IBM-compatible floppy disk for $4.00 (including tax &
shipping) from Robert A. Pease, 682 Miramar
Ave., San Francisco, CA 94112.
27. Scientific American, about $36/year. Call
(800) 333-1199 or (515) 247-7631.
28. Discover, Family Media, NY. About $27/
year. P.O. Box 420105, Palm Coast FL 32142;
(800) 829-9132.
29. Machine Design, a Penton Publication.
Write on letterhead to 1100 Superior Ave.,
Cleveland Ohio, 44114-2543.
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?
(Part 1)
What’s All This
Transimpedance
Amplifier Stuff,
Anyhow
Originally published January 2001
O
ne of the first things you learn
about operational amplifiers (op
amps) is that the op amp’s gain
is very high. Now, let’s connect a
feedback resistor across it, from
the output to the -input. When you
put some input current into the -input (also known
as the summing point), the gain is so high that
all of the current must go through the feedback
resistor. So, the output will be VOUT = -(IIN ×
RF). That’s neat (Fig. 1). While we used to call
this a “current-to-voltage converter,” which it
is indeed, it’s also sometimes referred to as a
“transimpedance amplifier,” where the “gain” or
“transimpedance” is equal to RF.
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(Part 1)What's All This Transimpedance Amplifier Stuff, Anyhow?
There’s a whole class
of applications in which
this configuration is
quite useful and important. An important case
is when you need an
op amp to amplify the
signal from a sensor,
such as a photodiode.
Photodiodes put out
current at high impedance (high at dc), but
often they have a lot of
capacitance. If you just let the photo
diode dump its current out into a resistor, there are two problems (Fig. 2). If
the sense resistor is large, then the gain
can be fairly large, but the response
will be slow and the time-constant will
be large: t = RL × CS. But
if you choose a small
sense resistor to get a
small t, the gain will be
low. The signal-to-noise
ratio (SNR) may also be
unacceptable. How can
you avoid poor gain and/
or poor response? Kay
garney? (That’s Nepali for
“What to do?”)
To avoid this terrible
compromise, it’s a good
idea to feed the photodi-
ode’s output current directly into the summing
point of a transimpedance amplifier (Fig. 3).
Here, the response time
is not RF × CS, but considerably faster. Plus,
the gain can be considerably larger, because now you can use
a larger RF. This helps
improve the signal-tonoise ratio too!
When you connect up the diode like
this, the first thing you realize is that the
darned thing is oscillating! Why? Well,
it’s well known that the input capacitance of an op amp (and its circuitry)
can cause instability when the op amp
is used with a feedback
resistor. You usually need
to add a feedback capacitor across RF to make
it stable. In the old days,
it was stated that:
CF × RF = CIN × RIN
So if you have a unity-gain inverter with RIN =
RF = 1 MO, and the input
capacitance of the op
amp is 10 pF, then you’re
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ELECTRONIC DESIGN LIBRARY
(Part 1)What's All This Transimpedance Amplifier Stuff, Anyhow?
supposed to install a feedback capacitor of 10 pF. That’s what people said for
years. The LF156 data sheet stated this,
and it still does. But that’s not exactly
true. A complete explanation is a bit
beyond the scope of this column, but in
practice you can usually get away with
a much smaller feedback capacitor. In
many cases, you can get a response
that’s improved by a factor of five or 10,
and still not get excessive (more than
5% or 10%) overshoot. In practice, you
have to tweak and optimize the feedback capacitance as you observe the
response.
The formula for the optimized amount
of CF is, if:
R
RF RFF + 1 ≥ 2 GBW × R × C
GBW
+ 1+ 1≥ 2≥ 2GBW
× R×F R
×FFC×S CSS
RIN
RINRIN
RF
× RF × CS
then: R + 1 ≥ 2 GBW
CS
IN
CS CS
CF =
CF C
=F =
RF
2 RFCRF + 1
2 2 RS + 1+ 1
CF =
RINRIN
IN
RF
but if:
+1
2
RF
R
RF RF + 1 < 2 IN
GBW × RF × CS
GBW
+ 1+ 1< 2< 2GBW
× R×F R
× FC×S CS
RIN
RINRIN
RF
+ 1 < 2 GBW × RF × CS
RIN
C
the feedback
capacitor
CF should
CS CSS
CF =
CF C
= F = GBW × RF
GBW
GBW
× R× R
CS F F
CF =
GBW × RF
be:
FOCUS ON: BOB PEASE ON ANALOG
Now, whenever you have an op amp
with a large CS, a large RF, and a small
CF, the noise gain will rise at moderate
frequencies. The definition of noise gain
is the reciprocal of the attenuation from
the output back to the -input. In other
words, if the attenuation is ZIN/(ZIN + ZF),
then the noise gain is 1 + ZF/ZIN.
At moderate frequencies, the ZF is
determined by RF, and ZIN is established
by CS. So, the noise gain will rise until
the frequency where the impedance of
CF becomes equal to RF. Then the noise
gain flattens out, typically at a large
number, such as 20, 40, or 80. We do
this because if the noise gain kept rising
at 6 dB/octave while the op amp’s gain
is rolling off at 6 dB/octave, the loop is
going to be unstable, and it will oscillate. The reason that we choose a small
value of CF is to make the noise gain
flatten out, make the loop stable, and
stop the oscillation and ringing (Fig. 4).
If you make CF = CIN, you can get the
noise-gain curve to stay flat as in line
A-E. It will be very stable but have a
very slow response. If you add no feedback capacitor, the noise gain will tend
to rise as per line A-B-C. This will cause
instability. Selecting a suitable small
value for CF can get the smooth results
shown by line A-B-D. Yeah, it’s as easy
as ABD to get fast, stable response by
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(Part 1)What's All This Transimpedance Amplifier Stuff, Anyhow?
picking a small CF. So, we have made
the feedback capacitance big enough
to stop the oscillation and minimize the
overshoot. Now what?
There’s a pretty good book by Jerald
Graeme (ex-Burr-Brown) on the topic of
the transconductance amplifier: Photodiode Amplifiers—Op Amp Solutions.
Jerry and I have definitely come to the
same basic conclusion. When you want
to optimize a transimpedance amplifier,
everything interacts. Therefore, every
time you compute the response and the
noise, and change any factor, the computations may change considerably.
There’s no simple or obvious way to
compute or optimize the performance.
The performance, in terms of response
or bandwidth, in terms of peaking or
overshoot, and in terms of noise or SNR,
is an extremely complicated, nonlinear, and
highly interacting function
of:
•the feedback resistor
•the source capacitance
•the feedback capacitance
•the desired bandwidth
•the desired gain factor
(which does predict the
full-scale output voltage)
•the voltage noise of the op amp
•the current noise of the op amp
•the input capacitances of the op amp
•and the gain-bandwidth product of the
op amp.
Jerry and I certainly agree on that.
Jerry’s book is well written, and for just
$55, it’s pretty much a bargain. I recommend it: ISBN = 0-07-024237-X.
But I also have worked on this general
problem many times over the years and
have several suggestions that go beyond Jerry’s book. More on this later.
There are several basic rules of thumb
that Jerry and I agree upon:
(A) You want to avoid an op amp with
high voltage noise (nV/vHz).
(B) You want to avoid an op amp with
high current noise (pA/vHz). (Most bipolar op amps have much higher current noise than FETs.) It’s
a rare case when an op
amp with bipolar input
transistors is better, except when RS is very low
or resistive (or in cases
where the input is capacitive but the bandwidth is
narrow).
(C) You usually want
to avoid an op amp with
large input capacitance.
Unfortunately, most data
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(Part 1)What's All This Transimpedance Amplifier Stuff, Anyhow?
sheets don’t properly
specify the op amp’s input capacitances, neither differential-mode nor
common-mode. But it’s
fair to assume that most
“low-noise” op amps
have a larger input capacitance than ordinary
op amps. You may want
to ask the manufacturer,
or you might just decide
to measure it yourself.
(D) Much of the noise of such a transimpedance amplifier is proportional to
vBW &215; CSOURCE × VN of the op amp.
So if you want to get low noise, you
must optimize very carefully. Specifically, begin by computing the im-pedance
ZS of your sensor at the
maximum frequency of
interest:
ZS = 1/2pFCS
For a good amplifier,
the voltage noise and the
current noise times ZS
should both be as small
as you can get. If one
of these noises is much
larger than the other, then
you’re probably far off
FOCUS ON: BOB PEASE ON ANALOG
optimum.
(E) If you have any
choice of what sensor
you employ, try to find a
lower-capacitance sensor. Furthermore, make a
low-capacitance layout
between the sensor and
the op amp.
If you want to get fast
response, low noise, or
wide bandwidth, Jerry’s
book offers some pretty good advice.
More on that later.
But Jerry didn’t include a list of good
op amps that have low voltage noise,
and/or low current noise, and/or low input capacitance.
Also, Jerry neglected to mention that
you can design your own
op amp with better, lower
voltage noise and better bandwidth. I mean,
op amps that you can
buy off the shelf cover
a wide array of cases
where they are optimized
for low VNOISE and low
INOISE, wide bandwidth,
low power drain, and
so on. But you can “roll
your own” surprisingly
easily and accomplish
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(Part 1)What's All This Transimpedance Amplifier Stuff, Anyhow?
even better performance for a specified application! I’m not proposing that
you design a complete op amp, but it’s
simple to just add a new low-noise front
end ahead of a suitable op amp.
The basic idea is to add a couple of
good low-noise FETs in front of an existing op amp. Most op amps don’t operate the front-end transistors as rich as
the output. Yet in a case like this, there’s
no reason at all not to run more current
through the front end than in the rest of
the op amp. My first pick is the 2N5486,
which has less than 1 pf of CRSS, but has
a lot of gms (4 millimhos)
and low voltage noise
(at IS = 3 mA). So for
my first design, I’ll just
put a matched pair* of
2N5486s in front of a decent wideband op amp,
such as the LM6171
(Fig. 5). What’s the voltage noise of this amplifier? We may be able to
get an average of 3 nV/
vHz, out to 10 kHz.
When you’re designing
an op amp, remember
this: adding gain is one
of the cheapest things
you can add. You on-ly
need to be careful about
how to give that gain away—to roll it off.
In this case, it’s easy. The R1-C1 network in Figure 5 just rolls off the gain for
a fairly smooth frequency response. To
achieve 2 MHz of bandwidth and a fairly
good, smooth 6-dB/octave rolloff, I suggest R1 = 75 O, and C1 = 100 pF as a
good place to start your design.
But now, look at the refinements in
Figure 6. We can roll off the amplifier’s
gain simply in two swoops. The low-frequency gain is rolled off by RX and
CX. Then after the gain rolls off flatly,
we roll it off some more by RY and CY.
When we are finished, it
should look something
like curve X in Figure 7.
This isn’t exactly rocket
science. We just want
to make it a practical
design. But this is a
whole system design.
You can’t very well design and optimize the
op amp alone. It’s the
op amp, the feedback
system, the noise filters,
and the post-amplifiers
that have to be considered and optimized all
together. My first-hack
proposals for these
damping/stabilization
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FOCUS ON: BOB PEASE ON ANALOG
components are:
have two, will three be
better? I’ll let you figure
RX = 5.1 kO, CX = 50
that out! But, yes, four or
pF
five may provide definite
improvements... or that
RY = 330 O, CY = 7.5
might not be the case.
pF
I won’t recommend
that you design your
The whole point beown op amp if you can
hind making your own
buy one that does the
op amp is that you do
job. But if the best one
not have to just build an
you can buy isn’t good
op amp with a smooth 6-dB/octave roll- enough, then there’s some hope here.
off, all the way out to a few megahertz.
Designing your own composite op amp
You can roll off the gain at a 6 dB/ocis not that hard, and not that expensive,
tave out to some intermediate frequen- even if you are going to build one or 10
cy, and then flatten out the gain. Then,
or 1000. The post-amplifier can be inexat a higher frequency, let it roll off some pensive. Of course, all of the basic demore in some vaguely controlled way.
signs will be somewhat different if you
This would make a lousy general-purare running on ±5-V supplies, or ±15-V
pose op amp, but it might be ideal for
supplies.
a case where the noise gain is rising,
Either way, it’s not that difficult, but the
such as in a transimpedance amplifidesign compromises are slightly difer. (Look at the old LM709. When you
ferent. Here, I just showed a couple of
choose the correct damping networks, it ±15-V applications. (The ±5-V designs
can provide a gain of 1000 out to some differ mostly by using a low-voltage, railhigh frequency like 1 MHz.)
to-rail-output op amp.)
Also note that I added a second pair
In future columns on this topic, I will
of 2N5486s to improve the voltage
comment on other aspects of design
noise. Yes, this will approximately dou- and optimization for transimpedance
ble the input capacitance. But if your CS am-plifiers.
is already large, this may easily improve
Meanwhile, try to avoid Tee networks
the signal-to-noise ratio. If it’s good to
in the feedback network. They often
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(Part 1)What's All This Transimpedance Amplifier Stuff, Anyhow?
cause poor signal-to-noise ratios. Next
time, I’ll explain that completely. Yes, a
Tee network might help you avoid buying 1000-MO resistors, but that’s only
okay when you have proven that the
noise is okay.n
teristics, such as noise, which is rarely
guaranteed.
P.P.S. I neglected to mention that any
resistor may have a built-in capacitance
of 0.3 to 0.8 pF. If you add that to any
imperfect layout, the capacitance could
be so big that you wish it were smaller.
RAP / Robert A. Pease / Engineer Good layout and good engineering can
easily cut the C to less than 0.2 pf. Fsor
*For this case, grade a good number example, make the feedback resistance
of 2N5486s into 20-mV bins of VS, with
out of three or four resistors in series,
VGD = 7 V, and IS = 3.8 mA. Take units
and install a shield land between the
out of the same bin for good matched
ends of the resistor.
pairs.
P.S. If you design in an op amp, try to
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?
What’s All This
FrequencyTo-Voltage
Converter Stuff,
Anyhow
O
Originally published July 1993
nce upon a time—my gosh, it was
30 years ago—a guy asked me if
I could show him how to make a
Frequency-to-Voltage converter
(FVC). Well, at that time, at George
A. Philbrick Researches, we knew
a lot about analog computers and we figured we
could convert almost any signal to any other form
or mode. So I designed a charge-dispenser made
of a voltage limiter, a capacitor, and diodes. I built
it up, and it worked pretty well.
And in 1964 we put this into the old Philbrick
Applications Manual.1 (Fig. 1).
The first amplifier has a limited output voltage.
The p-p voltage across the capacitor is pretty well
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What's All This Frequency-To-Voltage Converter Stuff, Anyhow?
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But the real improvement in this FVC is
the bleeder resistor, the 3.3 MΩ added
to the right end of the capacitor. If you
want a charge dispenser to dispense a
constant amount of charge, no matter
what the rep rate of the pulses, you can’t
let the voltage at the right end of the capacitor just sit there unattended. That’s
because it will be charged (through the
nonlinear impedance of the diodes) to a
different voltage, depending on how long
established:
you wait. The 3.3-MΩ resistor helps pull
charge off that node, so the p-p voltage
V p-p = 2Vz + 2Vd - 2Vd
is always constant at high or low speeds.
That is what’s required for good linSo, the charge (Q = C × V p-p) flows
earity—for minimum deviation from the
through the feedback resistor of the sec- straight line of:
ond amplifier. The output voltage will be,
on the average:
Vout = k × Fin + (error)
Vout = Rf × C × V p-p × f
Also, note the symmetrical Zener
clamp.2
A few years later, we got into the VoltAnother cute feature was the adaptive
age-to-Frequency Converter (VFC) busi- filter at the summing point of the second
ness. At the same time, I came up with
amplifier. The conductance of the diode is
an improved circuit for an FVC (Fig. 2).
linearly proportional to the current through
The input comparator is set up to acit, so the 1-µF capacitor gives an adaptive
commodate TTL signals, but if you put a time constant and helps filter the signal
resistor from the + input to -15 V, you can more at low frequencies, less at high freaccommodate symmetrical signals; a re- quencies. That’s the classical problem
sistor from the + input to ground will cut with most F-to-V converters: If you want
down the hysteresis and let you handle
to get low ripple, you get slow response
small signals.
due to the heavy filtering. If you want fast
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What's All This Frequency-To-Voltage Converter Stuff, Anyhow?
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have a frequency in the range 5 to 10
kHz and the frequency suddenly changes, you can get the output voltage to settle to the new level (within 1%) in about
40 ms—that’s about 200 cycles—yet the
ripple will be less than 5 mV p-p. That’s
about a 10:1 improvement over a single
R-C filter. Good, but not good enough for
some applications.
In 1979, I wrote another App Note5
showing how to use a phase-locked
loop to make a quicker F-to-V converter,
about 2 ms. That’s about 10 cycles of the
response, it’s hard to get low ripple.
new frequency—a further 20:1 improveAfter I left Philbrick, I joined National
ment.
and designed the LM131 voltage-to-freNot bad—but still not fast enough for
3
quency converter , using completely
everybody. For example, in a control
different ideas than any of the Philbrick
loop, you may need a voltage that repcircuits. It used Q = I × T, rather than the resents the frequency, and any delay or
Q = C × V employed by all of the Phillag in the information may cause loop inbrick ones. It didn’t need ±15 V; it could
stability. So a fast response can be very
run on +15 or +30 or +12 or +5 V—much important.
easier to apply. BUT, it still had the same
Recently, a guy asked me how to make
constraint when you used it as an F-to-V a 60-Hz FVC with quick response and
converter: If you want low ripple, it’s hard negligible lag or delay. I told him that the
to get fast response.
standard procedure is to use a fast clock
In 1978, I wrote an application note on and a digital counter. But the number of
how to improve the response time of an counts collected during one period is
FVC—in the Linear Apps Handbook.4
linearly proportional to the period of the
I showed how to cascade two or more
signal, and you might have to do some
fast Sallen-Key filters to give reasonably digital computations to convert that to a
quick response, yet filter out the ripple
signal representing the frequency. Then I
at 24 dB per octave. For example, if you realized that a “multiplying” DAC can be
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What's All This Frequency-To-Voltage Converter Stuff, Anyhow?
used to divide in a reciprocal mode.
I built it up and it worked. This Frequency-to-Voltage converter settles in one cycle of the frequency. Besides that, it uses
only a small number of parts (Fig. 3).
The digital logic generates a couple of
pulses at the time of each rising edge of
the incoming frequency (you could use
some kind of dual one-shot multivibrator,
but I didn’t have any of those around).
The first pulse loads the data from the
CD4040 into the DAC (the pulse also
disables the path from the clock to the
counter to avoid any confusion from rippling in the counter). Then the second
pulse resets the counter.
The MDAC has storage registers built
in, so the data from the counter is fed
right in to the DAC when the WRITE-2bar pulse is applied. The MDAC isn’t
connected in the normal way, with the
variable resistance in the input path. The
fixed resistor is in the input, and the impedance controlled by the Digital code
is connected as the feedback resistor.
This permits the multiplying DAC to act
as a divider, so the reciprocal function
is done neatly—not in the digital realm,
and not in the analog world, but on the
cusp between them. (More on this in a
few months). The LM607BN was chosen
for the op amp because you need low
offset. It’s cheap, Vos is only 25 µV typical
(60 µV max.), and you don’t need a trimmer pot.
The guy who asked me for this function
was quite pleased, as he said there are
several suppliers who are happy to sell
you this function for a couple hundred
dollars.
But it’s really not that big a deal. You
can do the whole thing yourself. All it
takes is just a few dollars worth of parts
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What's All This Frequency-To-Voltage Converter Stuff, Anyhow?
and a little labor.
The main limitation of this scheme is
getting a decent resolution on the output
voltage if you must cover a wide range
of frequencies. For example, if you have
to cover a 10:1 range, let’s say from 20
to 200 Hz, then you can only use a clock
frequency of 20 kHz with a 10-bit counter
(or the clock counter would overflow, giving unacceptable false answers).6 Then
at 60 Hz, the number of counts would be
just 333. The resolution would be only
one part out of 333, or one-third percent.
So, if 200 Hz is scaled for 10 V, 60 Hz
for 3 V, and 20 Hz for 1 V, then the FVC
can only resolve the difference between
60.18 Hz and 60.000 Hz—for example,
3.000 V and 3.009 V. The resolution at
200 Hz would be even worse, about 100
mV per step, because there are so few
COUNTS there.
If you need to get better resolution, you
can get a 4X improvement by using a
more expensive 12-bit MDAC and a 80
kHz clock. An 8-bit MDAC, even though
the price is right, can’t give much better
than 1% resolution, even if you use it in a
dynamic range of just one octave.
So, there’s the limitation to this counting method. But you have to admit it is
fast and has low ripple! (Of course, the
other limitation is that if you wanted a
fast computation for a 60-kHz frequency,
you might need a 20 or 80 MHz clock
and counter, not impossible, but not so
easy....)
RAP/Robert A. Pease/Engineer
p.s. And in an upcoming column, I plan
to write about Voltage-to-Frequency converters, too.
REFERENCES:
1. Teledyne Philbrick, Applications Manual,
1965, 1985; p. 95; (out of print).
2. How do you like that little Zener bridge circuit that’s inherently symmetrical in its swing?
As near as I can tell, it was first published by
NSC in an “Applications Corner” in Electronic
Design, p. 69, July 5, 1976. I sort of invented it
about 1971—has anybody ever seen it before
1976? I don’t think I have ever seen any patent
on it—and if there had been one, it would have
expired by now....
3. LM131/LM231/LM331 Voltage-to-Frequency
Converter data sheet; available on request.
4. National Semiconductor Linear Applications
Handbook; Appendix D, available on request.
5. Ibid., Application Note AN210.
6. That’s the function of the trick circuit shown
in Figure 3—to detect when the counter is
nearly full, shutting off the clock. This prevents preposterous outputs when the frequency approaches zero.
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?
What’s All This
Capacitor
Leakage Stuff,
Anyhow
Originally published March 2007
W
e all know that capacitors have
a shunt resistance (leakage) and
that leakage resistance should
be pretty easy to measure, right?
Wrong! I’ve measured a lot of
capacitors for short-term soakage
(dielectric absorption).
After the short-term soakage stops, it’s possible
(not easy) to measure the leakage. For example,
if you charge a good cap up to 9 V for a few
seconds, it will start discharging shortly for several millivolts. If you wait long enough, you may
see leakage slow down to a few millivolts per
hour. But you will see the long-term soakage. Is
that different from the short-time leakage? Maybe
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What's All This Capacitor Leakage Stuff, Anyhow?
not.
Now I will charge up some of my favorite low-leakage capacitors (such as
Panasonic polypropylene 1 µF) up to
9.021 V dc (a random voltage) for an
hour. I will read the VOUT with my favorite
high-input-impedance unity-gain follower (LMC662, Ib about 0.003 pA) and
buffer that into my favorite six-digit digital voltmeter (DVM) (Agilent/HP34401A)
and monitor the VOUT once a day for
several days.
Why did I choose 9 V? Because that’s
within the common-mode range of the
op amp and the DVM at highest resolution. I keep the input ball hook connected to +8.8 V dc between readings.
I also keep my left hand grounded to
+8.8 V.
DAY BY DAY
One of my e-mail colleagues had
been monitoring some good 0.1-µF
polystyrenes, and he was impressed
that they got down to a leak rate of better than a year after several months.
Well, I could see that my polypropylenes had their leak rate improve
even better than that in just a few days.
Refer to the list of voltages below:
Day 0: 9.0214 V
Day 1: 9.01870 V
Day 2: 9.01756 V
Day 6: 9.0135 V
Day 7: 9.0123 V
Day 8: 9.01018 V
Day 9: 9.00941 V
Day 11: 9.00788 V
Day 12: 9.00544 V
Day 13: 9.00422 V
The first day after soaking for an hour,
their leak rate was as good as 2.7 mV
per day. Not bad.
If you had a 1 million-MΩ resistor
across a 1-µF capacitor at the 9-V level, it would draw 9 pA, which would pull
down the capacitor 778 mV per day. All
the capacitor types I tested were better
than this, except some “oil-and-paper”
caps that supposedly had special qualities for audio signals.
If you had a 10-meg-meg resistance,
that would cause the cap to leak down
78 mV/day. With 100 meg-megs, it
would be 7.8 mV per day. Several good
capacitors soon began to leak slower
than that. After a mere week, some of
the best caps were leaking at a rate
down near 1 mV/day. Quite good. So,
what’s the big deal?
The big deal is that a time constant of
31.5 meg seconds is one year! So any
capacitor leaking less than 2.5 mV per
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What's All This Capacitor Leakage Stuff, Anyhow?
day is leaking at a tau (rate) of 10 years
or more. If you had to wait a few months
to get this leak rate, well, that’s not bad.
But achieving this leak rate in less than
two weeks is, I would say, quite good.
Less than a day? Spectacular.
So I’m finding that good polypropylene caps are better than the best (old)
polystyrenes, in terms of soakage or di-
electric absorption (early or late) and in
terms of leakage, early or late. Are Teflons any better? Not much. I may have
to buy a couple to find out. n
RAP/Robert A. Pease/Engineer
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?
What’s All This
Noise Gain Stuff,
Anyhow
Originally published February 2004
O
n my recent seminar tour, I asked
about 4000 engineers, “How many
of you use noise gain?” About 2%
of the people held up their hands.
Sigh. Noise gain is often not very
important or critical. That is one of
the reasons why op amps are so popular. They
tolerate almost any kind of feedback and any
noise gain. You can often see that by inspection.
But when it is important, things get interesting
pretty fast.
Let’s start with a definition of noise gain: Noise
gain is the reciprocal of the attenuation from the
output of an op amp (or any feedback loop) to
the input. In Figure 1, the attenuation is RIN/(RIN
+ RF). So the noise gain is (RF + RIN)/RIN. Or, (ZF +
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ELECTRONIC DESIGN LIBRARY
What's All This Noise Gain Stuff, Anyhow?
ZIN)/ZIN.
As I say in my lecture, it
is not a good idea to try to
memorize that, because that
just ensures that you will forget it. But you ought to recognize the cases where life
gets interesting.
If ZF is 100 kΩ and ZIN is 1 kΩ, the
noise gain will be about 100—or, technically, 101. This normally ensures
that the loop is very stable because
the noise gain crosses the amplifier’s
gain-bandwidth product at a place
where there is almost never any problem (Fig. 2a). However, the name “noise
gain” also reminds us that this really is
the gain for the noise! So figure 1 is often used as a noise-test circuit. And, as
noise does properly extend down to dc,
if the op amp has a VOS of 1 mV, then
the output offset will be 101 mV!
If RF/RIN is a small number,
and the noise gain is not very
high, the curve of Figure 2b
can apply. Most op amps are
happy with any noise gain
from 1 to 2000. But be cautious about any amplifier that
is specified for “Gain of 10
minimum.” Such an amplifier
has excessive phase shift at high frequencies, and it will surely oscillate if
FOCUS ON: BOB PEASE ON ANALOG
the noise gain is not at least
10 (or as stated). But, refer to
NSC’s App Note LB-42.
Reactances: If we add a
feedback capacitor across
RF, the noise gain can be
high at low frequencies, but
it will fall to 1 at high frequencies (Fig.
2c). Guess what? This cuts the output
noise—a very popular move!
However, if your application circuit
happens to have a lot of capacitance
on the summing point—which might
happen if you had a large photo diode
or a lot of cable capacitance—then the
noise gain will rise at higher frequencies. This can cause serious problems
because when the noise gain rises
(Fig. 2d), it tends to cross the op amp’s
gain-bandwidth product too steeply.
And, the 12-dB per octave slope will
cause severe ringing. This ringing or
oscillation can usually be
cured with a small feedback
capacitor. The nominal value for this capacitor will be
√2CIN/(GBWP × 2πRF). For
a typical circuit with a summing-point capacitance of
100 pF, the CF may need to
be 5 to 10 pF—not too huge.
Note, however, that while most of this
analysis is computed in the frequency
☞ REGISTER: electronicdesign.com | 42
ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Noise Gain Stuff, Anyhow?
domain looking at Bode plots, the evaluation of loop stability is normally done in
the time domain. Hit the input with a little step of current and see how well the
output settles without excessive ringing.
Often, an engineer proposes to use
a “tee network” for his feedback resistance to avoid having to buy a large-value resistor. I almost always say that
that’s a bad idea. Why? Because of the
noise gain! Either due to the noise, or
the bandwidth, or the drift, any tee network with a factor of more than 2 is usu-
ally a terrible idea. A noise gain of 10 or
100 or 1000 is often a terrible idea!
Next time you work with an op amp,
think about the noise gain. If it is a simple case, it won’t require more than a
moment. But if it is complicated, it might
require more than an hour! Or if you neglected to study the problem properly,
you’re in for days of grief! n
RAP/Robert A. Pease/Engineer
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Sep 5, 2000
?
What’s All This
Current Limiter
Stuff, Anyhow
Originally published September 2000
T
he other day I was studying a current
limiter using a basic current regulator,
which used an old LM317LZ (Fig. 1).
I did the design on this IC about 20
years ago, with a little advice from Bob
Dobkin. The LM317 was trying to force 40
mA into a white light-emitting diode (LED), so that
1.25 V across RS, the 30.1-ohms sense resistor,
would cause a 41-mA current to flow—if you
had enough voltage. (This is a GOOD standard
application shown in every LM317 data sheet.)
Because the output pin is 1.25 V above the
adjust pin, the current I = 1.25 V/RS will flow—if
there’s enough voltage.
It worked fine with an input voltage of 9 V, or 8
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ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Current Limiter Stuff, Anyhow?
V. But, of course, when it hit
7.4 V, it began dropping out
of regulation. This was the
basic design for a flashlight,
using a white LED and a 9-V
battery. The flashlight would
run with RS = 30 ohms for
a BRIGHT output, or 120
ohms for long battery life,
and still provide enough
light to hike with on a trail in
the pitch dark.
I pondered this for a while.
Would it do any better if I
put the load in the + supply
path of the LM317? How
about installing the load between A and B, and placing a jumper wire between
C and D? No, it would work
just as well—and just as
badly.
Then a few days later, I
realized—I could do a lot
better than that! Yes, an
adjustable current source,
such as an LM334N, might
do a tiny bit better than an
LM317. But see Figure 2.
The LM334N can regulate with not 1.25 V across
the sense resistor—but just
64 mV. So a 1.6-ohms re-
sistor will let you source
40 mA quite handily. And
the voltage across the
load does NOT have 0.7
V in series with it, so it
regulates down MUCH
better than Figure 1.
How much voltage is
across the transistor?
An ordinary 2N3906 will
keep working down to 65
mV. So you can force 40
mA into a white LED that
runs at 4.0 V, even down
to a 4.13-V battery voltage. If you want to put 20
mA into a series stack of
red LEDs, the conventional LM317 scheme
will light two LEDs with
a battery down to 6.3 V.
But the LM334 scheme
can drive three LEDs in
series, with the same
voltage. So it’s not a bad
circuit. This is a useful
trick, especially if you
have a load that’s floating, and isn’t grounded
to either supply.
THIS circuit doesn’t
have a low tempco. Its
output current increases
☞ REGISTER: electronicdesign.com | 45
ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Current Limiter Stuff, Anyhow?
at +0.33%/°C. But that’s plenty good
enough for many cases—like in a flashlight! If you need a better tempco than
that, we know several ways to do it.
Still, this will let you regulate the current
into a red LED down to 2.1 V of supply
voltage, or a white one down to 4.2 V—
MUCH better than 7.4 V.
OH—I almost forgot to say—the
LM334 sometimes needs a series RC
damper. My first guess was 2 µF and
22 ohms across the base-emitter of the
PNP. Actually, this circuit didn’t oscillate
or ring badly without an added capacitor, but the noise was a bit quieter when
I added a 2- or 10-µF electrolytic.
If you really want a low tempco, use
copper wire (magnet wire) for the resistor—that will cancel out nicely. You’ll
need 6 ft. of #34 gauge, or 10 ft. of #32.
Even if you did have a grounded load,
this circuit would regulate down to 5.4
V—considerably better than 7.4 V.
A few months earlier, I received a request for a somewhat-larger current
limiter, that would pass 280 mA (200 mA
ac rms) at 115 V ac, but no more than
300 mA. I thought a few seconds and
scribbled out the basic circuit of Figure
3.
I told the guy, “This will probably work,
but the FET has to be a big one, such
as an IRF640 or IRF740, with a large
heat sink, as it will have to handle 30 W.
And the current limit isn’t perfectly constant, or well defined, as the current limiter gets smaller at warm temperatures.
I don’t know if that’s what you want, but
that’s what you get. Tell me if that’s unacceptable. And if you nail the output
with a short-circuit load, it will probably
blow out.”
Later, I thought about this circuit.
Would it be possible to design in some
improvements, to avoid some of these
☞ REGISTER: electronicdesign.com | 46
ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Current Limiter Stuff, Anyhow?
disadvantages?
When a shorted load is applied, the
drain-source voltage will rise instantly to
+150 volts, and the gate-drain capacitance will try to pull the gate to +130 V.
That’s not so good! Maybe the 10 µF +
0.1 µF across the zener, with the Schottky diode D2 added, will help clamp the
gate to +15 V well enough to survive a
momentary short? (Fig. 4.)
The temperature coefficient problem
gets solved next. The VBE shrinks at
0.3%/°C in Figure 3. But the improved
circuit of Figure 4 gets a better tempco
of the current limit, by compensating for
that VBE tempco with the tempco of the
Schottky rectifier D3.
While this will provide a good current
limit with well-behaved loads, it may not
turn on the 2N3904 quickly enough with
severe loads. So I added the little diode
D4. This will give a short-term current
limit at perhaps 1 A, which will rapidly
fall to 0.3 A.
Now how about the long-term current
limit, in case of a short? That FET is going to get HOT!
Well—why don’t I add R6 and R7?
Normally, the voltage across the FET is
less than 0.1 V. When it goes into current limit, and the voltage across the
FET increases toward 100 V, the current
through R7 requires the current-limit
voltage across R5 to decrease. So in
case of a short, or a very heavy load,
the current limit folds back, and the heat
in the FET decreases. That sounds pretty good. But what if you need to start-up
a heavy load? That current through R6
would prevent the limiter from starting in
case of filament inrush current, or filter
capacitors that need to be charged.
Well—let’s add one more Band-aid.
Let’s add C4. This will let the limiter put
out a LOT of current for a short time, for
start-up. How much power loss? At 0.3
A, the burden (IR drop) is only 0.4 V for
the limiter circuit, plus 1.2 V for the rectifiers.
Well—that sounds like a good theory.
Does it work? Let’s see. When I draft☞ REGISTER: electronicdesign.com | 47
ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
What's All This Current Limiter Stuff, Anyhow?
ed this, I hadn’t yet built this. But I felt
it was well engineered, so I wasn’t too
surprised that it really did work as designed, when I built it.
I evaluated it very gingerly with light
loads, moderate loads, full loads, and
overloads. I ran it on a curve-tracer, and
turned up the voltage very carefully.
When I saw that it met every requirement, I dropped a short across the load.
The curve tracer grunted, but the regulator survived.
Then, I plugged that circuit into the
WALL and applied all kind of loads. After it survived every test, I shorted the
load again. I was pleased to see that it
ran cheerfully, it survived, and it didn’t
get very hot.
DISCLAIMER: Kids, DON’T just try
this at home. Working with high voltages can be lethal if you get sloppy. When
working on this circuit, turn the power
OFF and unplug the cord. If possible,
use an isolation transformer. Additionally, use a variac so you can turn up
the voltage gradually when starting out.
Wear safety glasses for all high-voltage testing. Keep one of your hands in
your pocket when actually working on
the circuit, to prevent a loop that could
send lethal currents through your chest.
Be careful of high voltages. If possible,
work on this ONLY when somebody is
around to give you first aid and call a
medic, in case of shock or explosion.
This high-voltage circuit is NOT guaranteed for anything. It does NOT come
with a guarantee of some safety margin.
Plus, it hasn’t been evaluated with any
loads other than resistive. Still—it’s a
pretty good place to start. /rap n
RAP / Robert A. Pease / Engineer
P.S. The last time I designed such
an “electronic fuse,” it blew up the FET
three times in a row when I applied
a short. I then gave up. This design
seems to work much better. But if I keep
decreasing the sense resistor by 3 dB
at a time, I wonder how far up the current limit can go before it starts blowing
up! But I’m not going to look at that tonight!
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ELECTRONIC DESIGN LIBRARY
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?
Part 1:What’s
All This Output
Impedance
Stuff, Anyhow
Originally published August 2008
A
few engineers were having a debate.
According to all the books, some of
them said, op amps are supposed to
have zero output impedance, or very
low. That means the output voltage
won’t change, just in case the output
current changes. Some older op amps had an
output impedance of 600 Ω or 50 Ω. So, the gain
of the amplifier won’t change just because the
load changes. That must be good.
But a couple of other engineers pointed out
that many modern op amps have a very high
output impedance. The advantage of high output impedance (for the op amp) is that when the
load gets lighter, the gain goes up. Is there any
☞ REGISTER: electronicdesign.com | 49
ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
Part 1: What's All This Output Impedance Stuff, Anyhow?
harm in having a higher
gain?
Sometimes it’s advantageous to have high gain,
and higher gain isn’t necessarily bad. This high output
impedance usually occurs
on rail-to-rail outputs, which
are “drain-loaded.” You
can’t have an emitter-follower or source-follower
output on a rail-to-rail output! If you did, it wouldn’t
swing rail-to-rail.
settle. Then we wait a few
milliseconds more, read
the changed voltage, and
average for a few milliseconds more (perhaps for 16
ms).
The voltage gain is related to the reciprocal of
those few microvolts of error. We measure gain for
a step, not for sines. So
does everybody else in the
industry. The settling time
is related to the amplifier’s gain bandwidth, not to its low-frequency “pole,”
A FOOLISH IDEA
which is just a fiction.
This same guy argued that when you
Let’s have a little more insight on this use an op amp with a gain of 2 million
gain stuff. “Nobody needs an op amp
or 10 million, “You can only see higher
with gain higher than 200k,” I once
accuracy when you have signals well
heard one foolish engineer say. “Bebelow 1 Hz.” Wrong again! If you put in
sides, nobody would want to measure
a +1-mV square wave at 10 Hz into a
an op amp’s gain at 2 million or 6 or
good op amp set up with feedback re10 million, because it would take many sistors for a gain of +1000, the amplifier
seconds of test time, at 0.1 Hz or slow- with 2M will settle in a few milliseconds
er, and nobody wants to pay for that test to 999.5 mV.
time.”
The output for the gain of 20M will
Wrong! We can test op amps for a
settle to 999.95 mV. If you had a gain of
gain of 1 million or 10 million in just a
just 200,000, it would settle to 995 mV.
few dozen milliseconds. We read the
Even with sine waves at 10 Hz, you can
summing- point voltage, put in a suitsee the difference in better gain accuable step, and wait a couple millisecracy. The output would be 999.5 mV or
onds for the summing-point voltage to
999.95 mV p-p, not 995 mV p-p, even at
lf an op amp can’t
put out much current,
no matter how you
try, then it won’t have
good performance
when you ask it to
drive heavy load
currents.
☞ REGISTER: electronicdesign.com | 50
ELECTRONIC DESIGN LIBRARY
FOCUS ON: BOB PEASE ON ANALOG
Part 1: What's All This Output Impedance Stuff, Anyhow?
10 Hz! Higher gain is better.
A SECOND OPINION
“If an op amp has a high gain at rated
load, and its output impedance is high,
then the gain gets higher when the rated load is taken off,” another engineer
argued. “The dc gain goes up, and then
the gain rolloff will be steeper, and you’ll
get more phase shift, which will make
the loop less stable.”
Well, I haven’t seen any op amps
whose rolloff gets steeper when the rated load is taken off, not for 35 years—
an amplifier whose phase-shift goes
bad. All modern op amps have Miller
feedback from the output, so when the
gain gets higher, the low-frequency
break just goes back even further—
even slower than 0.01 Hz. I’ve seen that
a lot. So, there’s no danger there if the
gain gets “too high.” A gain of 1M or
10M or 100M does no harm.
What is an example of an op amp
where the gain goes up when the rated
load is taken off? A lot? The LMC662 or
the LMC6482. These are CMOS amplifiers with ~ “rail-to-rail” output capability.
Of course, it makes a difference how
the output is controlled.
If there is a Miller integrator (capacitive feedback loop around the output
stages) to make the gain rise smoothly
at low frequencies, that can make the
gain smooth indeed. The gain could rise
smoothly at 6 dB per octave, even back
below 1 Hz or 0.1 Hz.
Okay, where is the proof of the pudding? What does the gain look like?
What does it do for a closed-loop gain
of 10? More in the next issue!
The real performance of an op amp
driving a load is also related to the gM,
or transconductance. If an op amp can’t
put out much current, no matter how
you try, then it won’t have good performance when you ask it to drive heavy
load currents. That’s generally true if
the ZOUT is high or low. You gotta have
some gM. (As the old trucker used to
say, you can’t climb hills with paper
horsepower.)
Let’s look at examples of both types. If
the load is lowered, does it hurt the gain
accuracy? The distortion? If the load is
lightened, does it hurt the gain accuracy ? The distortion? The settling time? In
the next column, we will put these questions to the test using the test circuit
shown for the “best” amplifier of 2006 n
RAP/Robert A. Pease/Engineer
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?
Part 2:What’s
All This Output
Impedance
Stuff,Anyhow
Originally published August 2008
W
hen I present seminars, I often ask
the members of the audience to
hold up their hands if they think
bipolar op amps have better gain
and linearity than CMOS. I get
a good majority of hands. But
neither is bad!
The good-old LM301A (well over 30 years old)
has a good gain of 260,000 at no load, with just
75 µV p-p of gain error while its output is swinging 20 V p-p (Fig. 1). What happens when we
put on a load? With its rated 2k load, the gain
falls and even reverses a little and becomes
nonlinear in its error.
The lower curve shown here (with a 1k load
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Part 2: What's All This Output Impedance Stuff, Anyhow?
1. Crossplots of gain error,
with test circuit per AN-1485:
LM301A, F= 5 Hz; output ± 10
V into 1 kΩ (lower trace); upper
trace, 75 µV p-p at 100 µV/div.;
lower trace., 48 µV p-p at 100
µV/div.
to exaggerate the error)
shows that the LM301A’s
nonlinearity becomes as
big as 48 µV p-p. This isn’t
due to low “gain” or gM, but to thermal
feedback from the output transistors to
the input transistors, which is caused by
imperfect layout of the temp-sensitive
input transistors.
Well, this does look lousy, but there
are mitigating factors. This thermal effect is most obvious at 0.2 to 20 Hz. At
frequencies above 150
Hz, this effect tends to go
away and smear out, and
at 1 kHz, you can hardly
see it. So, this isn’t a big
problem for audio amplifiers.
Also, if the amplifier isn’t
driving heavy load currents, this thermal problem
shrinks proportionately.
At light loads, it’s negligible. Even with a 4k load,
an LM301 can make a
unitygain inverter with a
nonlinearity of 1.2 ppm. At
lighter loads, it’s even better.
ON THE BENCH
Now let’s look at some
CMOS amplifiers with ~
rail-to-rail outputs. The
LMC662 is typical—one of
our first CMOS amplifiers (Fig. 2). Like
the LM301A, its gain degrades when
overloaded with 1 kO. But the nonlinearity (deviation from best straight line)
is only 18 µV p-p. That’s not bad for an
8-V, 8-mA p-p swing.
This isn’t thermal cross-talk, just a
matter of honest gain. The LMC662 has
four honest gain stages
to source current, but just
three stages to sink load
current. Still, its high output impedance causes
the gain to rise a lot when
lightly loaded. How high
2. LMC662, F = 6 Hz; output
±4 V into 1 kΩ (lower trace);
upper trace, 1µV p-p at 10 µV/
div.; lower trace, 27 µV p-p at
10 µV/div.
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Part 2: What's All This Output Impedance Stuff, Anyhow?
does it rise? Well, it seems
to rise higher than 4 million,
but the error is down in the
noise. As with the LM301A,
its distortion when driving a
4k load is about 1.2 ppm.
So, it really is possible to get low distortion with ordinary op amps. And, it’s easy to get exquisite linearity
with good, inexpensive amplifiers. For example, the
LMC6022 does better than 0.3 ppm (Fig. 3). That’s
pretty good for a micropower op amp. Its open-loop
ZOUT is above a megohm, but its closed-loop ZOUT is
below a milliohm! n
RAP/Robert A. Pease/Engineer
3. LMC6042, F = 0.6 Hz;
output ±4 V into 1 kΩ (lower
trace); upper trace, 2 µV p-p
at 20 µV/div.; lower trace, 6 µV
p-p at 20 µV/div.
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